Millikan Library DemonstrationEarthquake Engineering Research Laboratory

November 12, 2004

Case Bradfordcase@ecf.caltech.edu

http://ce.caltech.edu/~case

Overview

• Vibrational characteristics of Millikan library

– Forced vibration testing

• Changes in these characteristics

– Weather patterns

– Earthquake behavior

• Wave propagation / radiation damping

Robert A Millikan Memorial LibraryMillikan Library, one of the worlds most heavily researched

and instrumented buildings, has been studied extensively since its construction in the mid 1960s. Very stiff 9-story reinforced concrete building with one basement and no foundation piles.

A synchronized vibration generator (“shaker”) is installed on the roof for forced vibration tests.

USGS/Caltech Dense Instrumentation Network installed in 1998 -- 36 FBA-11 instruments, three horizontal stations on each floor (basement-roof) and three vertical stations in the basement. Digitized with two parallel systems:

– Mt Whitney digitizer/datalogger records triggered events

– Digitexx digitizer system streams real-time data to client computers

California Integrated Seismic Network (CISN) station MIK installed in 2001. Three-component episensor on 9th floor, Quanterra datalogger (this station is part of the COMET system).

Millikan EW modeshapes as determined from forced vibration testing

Resonance Curve Mode Shape

Modeshape movies: EW

Resonance at 1.1Hz

Modeshape movies: NS

Resonance at 1.7Hz

Modeshape movies: Torsion

Resonance at 2.4Hz

Historical Behavior of Millikan Library

Fundamental EW and NS modes of Millikan Library since construction.

Crosses indicate frequencies from forced vibration tests.

Circles indicate natural frequency estimates from recorded earthquake events, and numbers in italics are peak roof accelerations from the event (cm/s2).

Dashed lines represent the observed natural frequencies of the library, and the shaded region is the likely variance from such factors as weather conditions, weight configuration of the shaker used for forced vibration tests, and experimental error.

[Earthquake Abbreviations: LC: Lytle Creek, SF: San Fernando, WN: Whittier Narrows, SM: Santa Monica, NR: Northridge, BH: Beverly Hills, BB: Big Bear] - (Clinton, 2004)

One Week of DataA spectrogram of MIK (EW component) consisting of one week of data, with a

running FFT of one hour.

The vertical stripes in the spectrogram are quiet periods, the AC system turns off nightly from Midnight-4AM.

Dynamic Properties of Millikan Library vary on different timescales:

Weeks

Days

Years

Clinton, 2004

Santa Ana Windstorms

Big Bear Earthquake

M5.4 =119 22Feb2003

Forced Vibration Testing

Santa Ana Windstorms

Big Bear Earthquake

M5.4 =119 22Feb2003

Forced Vibration Testing

Weather Conditions

Examining data from extreme weather events, we can see that both strong winds and heavy rains can affect the dynamic behavior of the library.

Heavy winds have been observed to temporarily decrease all fundamental frequencies by 2-4%. The building recovers immediately after the wind event.

Rainfall causes an increase in the EW and Torsional fundamental frequencies of 3-5%, the NS mode is less strongly affected by rain. Rain effects persist for 1-2 weeks, gradually reverting to pre-rain levels.

Clinton, 2004

Earthquake Movies

Long Distance Studies

J. Favela, 2004

Long Distance Studies

J. Favela, 2004

• Finite element modeling of the Pasadena area -- excitation from our forced vibration tests

Review

• Vibrational characteristics of Millikan library– Mode shapes and natural frequencies

– Forced vibration behavior

• Changes in these characteristics– Weather patterns: Rain and wind cause large changes in

stiffness

– Earthquakes: Millikan is surprisingly nonlinear for small to moderate sized events

• Wave propagation / radiation damping